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rfc-editor.org/rfc/rfc1123 ("Requirements for Internet Hosts -- Application and Support") has one cite for "hostname", but only in the context of the name of a reference doc. The other 20 mentions are as two words. I think "hostName" (sic) although common usage, is largely down to the IT industry liking to variable-name-ify terms. Doesn't mean it's not accepted usage though.